Antithyroid activity of 6-chloropurine

Abstracts

Antithyroid activity of 6-chloropurine was determined, in vitro, by studying its complex formation with iodine spectrophotometrically. The complex exhibited a 1:1 stoichiometry. Formation constant (Kc) of the complex was found to be 7.652 × 10³ L mol-1, which indicated good antithyroid potential of 6-chloropurine. For further verification, changes in the thyroid hormone levels of the male rabbits were observed, in vivo, after administering the dose through intra peritoneal (i.p.) injections for 14 days and measuring free triiodothyronine (T3) and tetraiodothyronine (T4) levels in the blood serum using radioimmunoassay technique. The blood assays showed decrease in free T3 and T4 levels in the treated animals as compared to the control ones thus confirming fairly good antithyroid activity of 6-choloropurine.

6-chloro-9H-purine; iodine; antithyroid activity; charge transfer complex; thyroid hormones

A atividade antitireóidea do 6-cloropurina, in vitro, foi determinada espectrofotometricamente por estudos de formação de seu complexo com iodo. O complexo mostrou estequiometria de 1:1. A constante de formação do complexo encontrada foi 7,652 × 10³ L mol-1, o que indicou um bom potencial de 6-cloropurina, como agente antitireóideo. Posteriores verificações mostraram mudanças nos níveis hormonais tireóideos em coelhos machos, in vivo, após administração da dose via intraperitoneal (i.p.) durante 14 dias e medindo os níveis de triiodotironina (T3) e tetraiodotironina (T4) livres, na circulação sanguínea, usando-se a técnica de radioimunoensaio. As análises de sangue mostraram decréscimo nos níveis de T3 e T4 livres em animais tratados, quando comparados com os controles. Desta maneira, confirmou-se uma boa atividade antitireóidea do 6-cloropurina.

ARTICLE

Antithyroid activity of 6-chloropurine

Ismat Fatima^I; Munawar A. Munawar^I,*; Misbahul A. Khan^I; Asmatullah^II; Afia Tasneem^III; Muhammad Khalil^II

^IInstitute of Chemistry and Department of Zoology, University of the Punjab-Lahore, Pakista

^IIDepartment of Zoology, University of the Punjab-Lahore, Pakista

^IIICentre for Nuclear Medicine, Mayo Hospital-Lahore, Pakistan

ABSTRACT

Antithyroid activity of 6-chloropurine was determined, in vitro, by studying its complex formation with iodine spectrophotometrically. The complex exhibited a 1:1 stoichiometry. Formation constant (K_c) of the complex was found to be 7.652 × 10³ L mol^-1, which indicated good antithyroid potential of 6-chloropurine. For further verification, changes in the thyroid hormone levels of the male rabbits were observed, in vivo, after administering the dose through intra peritoneal (i.p.) injections for 14 days and measuring free triiodothyronine (T₃) and tetraiodothyronine (T₄) levels in the blood serum using radioimmunoassay technique. The blood assays showed decrease in free T₃ and T₄ levels in the treated animals as compared to the control ones thus confirming fairly good antithyroid activity of 6-choloropurine.

Keywords: 6-chloro-9H-purine, iodine, antithyroid activity, charge transfer complex, thyroid hormones

RESUMO

A atividade antitireóidea do 6-cloropurina, in vitro, foi determinada espectrofotometricamente por estudos de formação de seu complexo com iodo. O complexo mostrou estequiometria de 1:1. A constante de formação do complexo encontrada foi 7,652 × 10³ L mol^-1, o que indicou um bom potencial de 6-cloropurina, como agente antitireóideo. Posteriores verificações mostraram mudanças nos níveis hormonais tireóideos em coelhos machos, in vivo, após administração da dose via intraperitoneal (i.p.) durante 14 dias e medindo os níveis de triiodotironina (T₃) e tetraiodotironina (T₄) livres, na circulação sanguínea, usando-se a técnica de radioimunoensaio. As análises de sangue mostraram decréscimo nos níveis de T₃ e T₄ livres em animais tratados, quando comparados com os controles. Desta maneira, confirmou-se uma boa atividade antitireóidea do 6-cloropurina.

Introduction

Propylthiouracil and methimazole are the only two widely used antithyroid drugs.¹ Undesirable side effects associated with these drugs have been observed for the past years.² This scarcity of antithyroid drugs motivated the search for new antithyroid agents. Consequently, antithyroid activity of a number of synthetic and natural compounds was studied.³ Synthetic antithyroid agents may either act by making stable complexes with iodine or by interfering with thyroid peroxidase (TPO) enzyme. In the first mechanism they make stable electron donor-acceptor complexes with molecular iodine produced by the oxidation of iodides, diverting the oxidized iodides away from thyroglobulin and ceasing the biosynthesis of thyroid hormones (T₃ and T₄). The formation constant (K_c) is indicative of the strength of complex. Higher the K_c stronger will be the complex. While in a second proposed mechanism, TPO is inactivated by the action of antithyroid agents, which block its active Fe³⁺. The ability of TPO, to oxidize the iodides and coupling monoiodotyrosine and diiodotyrosine residues of thyroglobulin to produce thyroid hormones, is greatly reduced. Lacto peroxidase (LPO), an analog of TPO is used to study the enzyme inhibition ability of the antithyroid compounds, in vitro. However, it has been observed that a number of drugs like levamisole, tetramethylthiourea, tetrahydrozoline and phenothiazines etc, which did not inhibit peroxidase activity showed high complexing tendency with iodine (obvious from high K_c values) and also exhibited strong antithyroid activity in vivo.⁴ Electron donor properties of purines have been reported by Pullman and Pullman.⁵ The formation of charge transfer band on mixing chloranil and 1,3,5-trinitrobenzene with adenine and caffeine in dimethylsulfoxide (DMSO) was studied by Beukers and Szent-Gyorgyi,⁶ while Machmer and Duchesne⁷ described the formation of a donor-acceptor complex between chloranil (acceptor) and adenine and guanine (donors). Helm and co-workers,⁸ were the first to report the structural and spectral information about a charge transfer (CT) complex of purine, stabilized primarily, by donation of n electrons from ring nitrogen. Single crystals of iodine complex of 9-cyclohexyladenine were obtained by allowing iodine to diffuse into a CCl₄ solution of 9-cyclohexyladenine. The complex nature was stated to be 1:1. Likewise, 1,7-dialkylxanthines and some dimethylxanthines were reported to have antithyroid activity,⁹ whilst adenine and guanine also exhibited low level antithyroid activity. Similarly, charge transfer complexes of purines with bromanil and p-benzoquinone have also been reported.¹⁰ There exists a good correlation between antithyroid activity, in vivo, and formation constant (K_c) of complexes of different organic compounds with iodine. K_c> 100 L mol^-1 indicates presence of considerable antithyroid activity.¹¹

The above mentioned and many other similar facts demonstrated the need to check purines for potential antithyroid activity and 6-chloro-9H-purine (Figure 1) was the first choice in this regard because in an initial spectrophotometric analysis in the laboratory using method of continuous variations",¹² 6-chloropurine-iodine complex showed 1:1 stoichiometry. Lang¹³ described a simple method to determine K_c of such complexes. Though K_c can be used as an indicator of antithyroid activity yet the decisive factor is in vivo testing of the compound. Nevertheless, K_c can help in predicting and selecting compounds for in vivo studies, which are conducted on animals (mice, rats and rabbits) thus reducing the investigation cost. The hormone levels are determined after administering the compound to the animals, in vivo, for a certain period of time. Variation in hormonal levels indicates antithyroid effect of the compound.

Experimental

Iodine was obtained from Merck (suprapur) bisublimed and was kept in dark in a dessicator containing P₂O₅. DMSO of spectroscopic grade was obtained from Merck, was dried over calcium hydride, distilled under reduced pressure and stored over type 4A molecular sieves. 6-Chloropurine was obtained from Sigma Aldrich. Spectra were recorded on a double beam UVD-3500 spectrophotometer (Labomed Inc.). Radioimmunoassay (RIA) kits for free T₃ and T₄ were obtained from Immunotech (France) and blood assay was carried out at Centre for Nuclear Medicine Lahore (Pakistan).

In vitro

Complex formation with iodine is an indication of antithyroid activity of a compound. K_c proved to be a useful parameter in this regard. In vitro antithyroid activity of 6-chloropurine was assessed spectrophotometrically by studying its complex formation with iodine. Solutions of iodine and 6-chloropurine were prepared just before the start of experiment by diluting gravimetrically prepared stock solutions in DMSO. Iodine concentration was kept constant (3×10^-5 mol L^-1), while the concentration of 6-chloropurine was varied between 1×10^-4 mol L^-1 and 1×10^-3 mol L^-1. The reaction was carried out directly in the spectrophotometric cell by mixing 1.5 mL solutions each of 6-chloropurine (donor) and iodine (acceptor). Spectra were recorded immediately on double beam UV Visible spectrophotometer, which showed formation of a charge transfer (CT) complex at a different λ_max than the reactants.

In vivo

Male rabbits weighing 1.35-1.40 kg were kept in animal house (Department of Zoology, University of the Punjab, Lahore) for in vivo study of variation in thyroid hormone (T₃,T₄) levels on treatment with 6-chloropurine. They were divided into three groups of five animals each and were provided with green fodder with water ad libitum. The animals were kept under standard laboratory conditions. First group (treated) of five rabbits received daily dose of 20 mg kg^-1 of 6-chloropurine solution in DMSO via i.p. injection for 14 days. Second group (vehicle control) received the equivalent dose of DMSO daily while the third (control) was kept untreated for the same period of time. Blood samples were collected from all three groups by dilating and puncturing veins of ears. The dose application and sample collection were performed under standard sterilized conditions.¹⁴ The levels of free T₃ and T₄ from the samples were determined using radioimmunoassay technique.

Results and Discussion

Both iodine and 6-chloropurine absorbed in UV region. However, the formation of complex led to the appearance of a CT band. This band is characteristic of the complex formation. It was quantified by placing a solution of pure donor at the same concentration in the reference beam, and then by subtraction of absorption due to iodine. Complex formation was observed between 215 and 225 nm with λ_max at 225 nm. The absorbance values at 215 nm, 220 nm and 225 nm wavelengths were recorded for different concentrations of 6-chloropurine and constant concentration of iodine (Table 1).

K_c was determined using Lang's method,¹⁵ which has been successfully used to determine the formation constants of 1:1 stoichiometric complexes,

where [I₀] is the initial concentration of iodine and [P₀] is the initial concentration of 6-choloropurine and [C] is the concentration of complex:

where d_c is the absorbance of the complex and ε_c is extinction coefficient. Equation 1 can be re-written in the form:

where Y= [I₀][P₀]/d_c and X= [I₀] + [P₀] - d_c/ε_c.

Equation 3 is an equation of a straight line with slope 1/ε_c and Y-intercept 1/(K_cε_c).

With fixed iodine concentration and different compound concentrations, different values of d_c and hence Y were obtained. X contained unknown values of K_c and ε_c. Iteration and linear regression were used to solve this equation. An initial value of ε_c was assigned and value of X was calculated. From the slope of the line, new value of ε_c was determined and used in the same process until the ε_c as well as K_c converged to particular values. Iterations were carried out by developing a simple computer program in the laboratory. The final values of X and Y calculated, by the same iterative method were recorded (Table 2). The XY scatter was plotted to which linear regression curve gave the best fit (Figure 2), with coefficients of determination (R² > 0.99)K_c was calculated for all three wavelengths (Table 3) and mean value was found to be 7.652 × 10³ L mol^-1, which indicated fairly good antithyroid activity of 6-chloropurine.

6-chloropurine (donor) did not absorb in visible region. When a solution of iodine was added to the donor solution, the characteristic band of the halogen shifted towards shorter wavelength (Δ = 15 nm). Spectra were recorded with different concentrations of 6-chloropurine and a fixed concentration of iodine (I₂ = 3×10^-5 mol L^-1) between 300-800 nm at 20 ºC. The absorbance peak of the complex was determined by placing a solution of iodine of the same concentration in the reference beam.

Circulating hormone levels i.e., free T₃ and T₄ of all three animal groups were determined after 14 days using RIA technique (Table 4). In hyperthyroidism, i.e. over active thyroid gland, thyroid hormones (T₃ and T₄) concentrations are increased in the blood and vice-versa. Therefore, a corresponding decrease in free T₃ and T₄ levels in the blood serum of the treated animals with respect to control will indicate antithyroid effects of the injected compound (6-chloropurine). The blood assays of the animals showed 8% and 20% decrease in average levels of free T₃ and T₄ in the treated animals as compared to the control ones, while the pertaining decrease with respect to the vehicle control was 31% and 26% respectively (Figure 3). The blood assay data was also checked statistically using the famous Student's t test; to further confirm the decrease in hormonal levels of the treated category with respect to the control and vehicle control categories. In this regard, the proposed null and alternative hypotheses were:

Null hypothesis (H₀): µ_c - µ_t = 0;

where, µ stands for population mean, while 'c' and't' for control and treated groups respectively.

Alternative or research hypothesis (H_A): µ_c - µ_t > 0 or µ_c > µ_t

where

_c and _t are the mean of control and treated values respectively and n is the total number of samples and S_x (standard error of the means) is given by:

S_x = √ [{Σ (X_ci - _c) + Σ (X_ti - _t)}/ (n_c + n_t - 2)]; n_c and n_t are total number of samples in control and treated classes respectively, which in this case are equal.

"t" values were calculated from the assay results and were compared with the critical values of t'_0.001(8) (significance level = 0.001, degree of freedom = 8) from the table, which rejected the null hypothesis, thus accepting the research hypothesis. Therefore, significant decrease in the T₃ and T₄ levels of the treated animals, as compared to the control and vehicle control groups was recorded, which confirmed the antithyroid effects of 6-chloropurine. However, the vehicle control group showed a rise in the hormonal levels as compared to the control group.

Conclusions

Antithyroid activity of 6-choloropurine was proved both in vitro and in vivo. The blood assay of treated animals compared to the vehicle control animals showed a marked decrease in hormonal levels. K_c proved to be a good indicator of antithryoid activity for1:1 stoichiometric complex of 6-chloropurine with iodine.

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Submitted: September 3, 2009

Published online: May 20, 2010

Publication Dates

• Publication in this collection
  13 Sept 2010
• Date of issue
  2010

History

• Accepted
  20 May 2010
• Received
  03 Sept 2009